The rate of coronary occlusion during acute myocardial infarction in man must vary widely, and it is not known how this variation may influence the severity of ischemic injury. This animal study demonstrates a 30% greater increase in coronary collateral blood flow when total occlusion was preceded by a 45 min gradual progressive stenosis rather than abrupt occlusion. Thus, interventions which can slow the rate of progression of coronary occlusion may reduce the severity of resulting myocardial ischemic injury by allowing greater collateral flow.